A heated calender roll is used in many industrial processes, for instance paper-making, plastic-sheet embossing, and panel production. It typically comprises a body having a cylindrical outer surface centered on a roll axis and a pair of axially spaced end or hub members in which the ends of the body are set. These hubs in turn are held in bearings or journals and are connected to drive belts or gears in the case of driven calender rolls.
In order to heat a calender roll it is standard as described in German utility models 8,410,839 and 8,436,564 and German patent document 2,750,047 to form the roll body with an array of flow passages and to provide means for pumping a heat exchange medium such as steam or hot oil through these passages. Since the medium must pass through the bearing or joint to move between the equipment heating the medium and the roll, such an assembly can get quite complex. The hot medium, frequently 100.degree. C.-200.degree. C., presents a danger for nearby workers if a supply hose ruptures and is generally difficult to conduct and manage.
It has therefore been suggested in French patent 1,343,136 to use a hollow-cylindrical roll body that is internally fitted with electrically powered heating elements of the resistive type. Heat is conducted by a liquid heat-exchange medium from the elements to the roll-body surface. Thus this liquid is wholly confined in the calender roll and does not need to pass through the rotating joint, considerably simplifying the equipment.
Since oil or water is not a truly efficient heat-exchange medium at the above-given temperatures, typically because there is some phase or composition change on heating, it has been suggested in German patent document 4,016,823 to use as the heat-exchange medium a liquid metal that melts at a temperature slightly below the operating temperature of the roller. This arrangement is particularly susceptible to leakage since most molten metals are very hard to contain.
Similarly German patent document 4,033,986 proposes using water in the roll in special chambers that move it convectively and that ensure its condensation and boiling under controlled circumstances. The problem with this arrangement is that it limits the temperature range for use of the roll, in particular setting a relatively low upper limit.